>>29302341The way I interpret it is that only Cubone "wears" a skull. That is, the skull isn't part of its body and is merely an accessory. Upon evolving, though, the skull actually fuses to Marowak's own and becomes part of its head. I don't think of it as pure bone anymore, maybe some combination of plating and cartilage that is capable of movement. Otherwise Marokwak would be incapable of closing its eyes.
I mean, officially Marowak's head has no skin at all, does it even have eyelids?